1. Field of the Invention
The present invention relates to an aberration correcting apparatus for correcting aberration of a light beam in an optical system.
2. Description of the Related Art
Optical discs such as a CD (Compact Disc) and a DVD (Digital Video Disc or Digital Versatile Disc) are well known as information recording media for optically recording and reproducing information. Furthermore, there are various types of optical discs, for example, optical discs for reproduction only, write-once optical discs on which only additional recording can be done, and rewritable optical discs on which information can be erased and re-recorded.
Research and development are in progress for developing high-density optical discs and optical pickups and information recording and/or reproducing apparatuses (hereinafter referred to as recording/reproducing apparatus) applicable to the high-density optical discs. In addition, research and development have also been pursued for developing optical pickups and information recording/reproducing apparatuses having the capability to be used for optical discs of different types.
A method of using with the high-density discs by increasing a numerical aperture (NA) of an objective lens provided in the pickup apparatus has been considered. Another method is the use of a light beam having a shorter wavelength.
However, the aberration of the light beam caused by an optical disc is increased as the numerical aperture NA of the objective lens is increased or a light beam having a shorter wavelength is used. This makes it difficult to improve performance accuracy of the recording/reproduction of information.
For example, when an objective lens having a large numerical aperture is used, the amount of birefringence distribution, which depends on the incidence angle, is increased at the pupil surface of the optical disc, since the range of the incidence angle of the light beam to the optical disc is increased. This creates an aberration problem due to the birefringence becoming more influential. In addition, the effect of aberration produced by error in the thickness of a disc cover layer for protecting the disc recording surface increases.
Such an aberration correcting apparatus is disclosed, for example, in Japanese Patent Laid-Open Publication Kokai No. 2000-131603. The apparatus includes two groups of lenses (or beam expander) to correct spherical aberration of the light beam produced at the optical disc by varying a distance between the lenses along an optical axis of the light beam. Another aberration correcting apparatus disclosed in Japanese Patent Laid-Open Publication Kokai No. H10-106012 includes a coupling lens for correcting the spherical aberration of the light beam produced at the optical disc by varying the distance between a laser source and the coupling lens along the optical axis of the light beam.
However, the object lens and the aberration correction lens are separately driven so that displacement occurs between the lenses in the above-described aberration correcting apparatuses. The aberration is corrected coarsely to ensure tolerance to the displacement, and thus unremoved spherical aberration remains. The residual spherical aberration obstructs signal-recording/reproducing operation, particularly in an optical disc having large thickness error of covering layer or a multi-layered recording disc having a plurality of recording layers.
As another conventional technique for reducing the aforementioned aberration, an optical pick-up apparatus having a liquid crystal element for aberration correction is proposed. An example of such aberration correction element is disclosed in Japanese Patent Laid-Open Publication Kokai No. H10-269611. The aberration correction element has a plurality of phase adjustment portions formed concentrically and predetermined voltages are applied to the phase adjustment portions to adjust orientation of the liquid crystal molecules. Thus, the aberration of a light beam can be corrected. It is, however, necessary to increase a thickness of the liquid crystal and apply a large voltage to the liquid crystal in order to correct an aberration larger than a wavelength of the light source. Furthermore, a number of phase adjustment portions must be provided to correct various magnitudes of wave front aberrations. Therefore, a large number of electrodes must be provided, resulting in complicated wiring. Accordingly, the aberration correction element has been difficult to be sophisticated in terms of size, thickness and response. Also, it is a problem that increase of the liquid crystal thickness produces frequency response deterioration of the aberration correction element.